Most drilling rigs have derricks with crown blocks generally centered over a vertical line extending upward from the well bore. A traveling block is suspended below the crown block and moves along the vertical line to lift an lower hook loads, which often include a drill string suspended in the well. Both the crown block and the traveling block have several sheaves situated side-by-side to turn about a common axis.
A draw works has a powered reel of cable, usually called a line, that extends up to the crown block and is reeved about the crown block and traveling block sheaves to develop the load carrying total capacity needed, and the line finally extends from the crown block to the rig floor where it is secured, usually by a few wraps around an anchor sheave. A line clamp is usually situated between the anchor sheave and the reserve line reel. Due to the coefficient of friction, the friction of the multiple wraps around the dead line anchor may leave little or no load on the line clamp.
The line from the draw works is called a fast line and the line to the clamp is called a dead line. The load on each of the reeved lines is the hook load plus the tare loads related to the traveling block, divided by the number of lines extending upward from the traveling block. For practical estimates, the load on the live line and the dead line is equal to the load on each length of reeved lines. The crown block is loaded by two more lines, the fast line and the dead line, than the number of lines extending from the traveling block.
Operationally, the dead line serves a maintenance purpose. The lines wear more in some locations, along the length, than in other locations. To get the maximum service from the investment in lines, the lines are periodically drawn from an ever-present reserve line spool through the dead line clamp, along the sheave system and onto the draw works spool. To accommodate the added line, some line is cut from the end anchored on the draw works spool. That action puts the more worn line lengths in locations less subject to wear, and the line system deteriorates over the length more evenly. To add safety and convenience to the dead line clamp arrangement, a dead line anchor sheave is usually placed between the clamp and the length of dead line extending to the crown block. The anchor sheave is usually locked, even if it has release means to allow it to rotate during the above described maintenance line feeding operation. Further, the dead line usually activates a hook load indicator. There is considerable resistance to design changes related to the long term successful use of the above described dead line anchoring system.
The derrick is designed and certified for a specific vertical load on the crown block. When being designed, or redesigned, a derrick can qualify for more hook load if there is no dead line to be considered as part of the crown lock load. To aid in the problem description it is convenient to consider a six sheave traveling block. Seven sheaves are needed in the crown block sheave cluster. The sheave carrying the dead line (the seventh sheave) does not rotate during movement of the traveling block. The seventh sheave does rotate during the maintenance activity, to feed line through the system from the reserve line spool. If the seventh sheave is changed to accept more wraps of line (usually 3 to 5), then locked to prevent rotation when normal activity, such as drilling, takes place there is little if any load on the dead line. The crown block load is thus reduced by about one part in fourteen. That load reduction can be in the nature of seventy thousand pounds. A specific derrick can then qualify for a hook load increase of that amount.
To make such changes in the dead line anchor system, safety, hook load indication, and the maintenance line feed has to be considered. This invention addresses those problems.
These and other objects, advantages, and features of this invention will be apparent to those skilled in the art from a consideration of this specification, including the attached claims and appended drawings.